Transcript Streams and File I/O

Walter Savitch
Frank M. Carrano
Streams and File I/O
Chapter 10
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Objectives
• Describe the concept of an I/O stream
• Explain the difference between text and
binary files
• Write data, including objects, to a file
• Read data, including objects, from a file
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Overview: Outline
• The Concept of a Stream
• Why Use Files for I/O?
• Text Files and Binary Files
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
The Concept of a Stream …
• Use of files
 Store Java classes, programs
 Store pictures, music, videos
 Can also use files to store program I/O
• A stream is a flow of input or output data
 Characters
 Numbers
 Bytes
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
… The Concept of a Stream
• Streams are implemented as objects of
special stream classes
 Class Scanner
 Object System.out
• Figure
10.1
I/O Streams
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Why Use Files for I/O?
• Keyboard input, screen output deal with
temporary data
 When program ends, data is gone
• Data in a file remains after program ends
 Can be used next time program runs
 Can be used by another program
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Text Files and Binary Files …
• All data in files stored as binary digits
 Long series of zeros and ones
• Files treated as sequence of characters
called text files
 Java program source code is one example
 Can be viewed, edited with text editor
• All other files are called binary files
 Movie files, music files, Java class files
 Access requires specialized program
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
… Text Files and Binary Files
• Figure 10.2 A text file and a binary file
containing the same values
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Text-File I/O: Outlline
• Creating and Writing to a Text File
• Appending to a Text File
• Reading from a Text File
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Creating a Text File …
• Class PrintWriter defines methods
needed to create and write to a text file
 Must import from package java.io
• To open the file
 Declare a stream variable for referencing the
stream
 Invoke
PrintWriter constructor, pass file
name as argument
 Requires
try and catch blocks
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
… Creating a Text File …
• File is empty initially
 Once created, it may be written to using
method println
• Data goes initially to a “buffer” in memory
 When the buffer is full, data goes to the file
• Closing a file empties buffer, disconnects
from stream
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
… Creating a Text File …
• View sample program, listing 10.1
class TextFileOutputDemo
Sample
screen
output
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
… Creating a Text File
• When creating a file
 Inform the user of ongoing I/O events,
program should not be "silent"
• A file has two names in the program
 File name used by the operating system
 The stream name variable
• Opening, writing to file overwrites preexisting file in directory
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Appending to a Text File
• Opening a file new begins with an empty file
 If already exists, will be overwritten
• Some situations require appending data to
existing file
• Command could be
outputStream =
new PrintWriter(
new FileOutputstream(fileName, true));
• Method println would append data at end
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Reading from a Text File …
• Note text file reading program, listing 10.2
class TextFileInputDemo
• Reads text from file, displays on screen
• Note
 Statement which opens the file
 Use of Scanner object
 Boolean statement which reads the file and
terminates reading loop
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
… Reading from a Text File …
Sample
screen
output
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
… Reading from a Text File
• Figure 10.3 Additional methods in class
Scanner
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Techniques for Any File
• The Class File
• Programming Example: Reading a File
Name from the Keyboard
• Using Path Names
• Methods of the Class File
• Defining a Method to Open a Stream
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
The Class File
• Class provides a way to represent file
names in a general way
 A File object represents the name of a file
• The object
new File ("treasure.txt")
is not simply a string
 It is an object that knows it is supposed to
name a file
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Programming Example
• Reading a file name from the keyboard
• View sample code, listing 10.3
class TextFileInputDemo2
Sample
screen
output
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Using Path Names
• Files opened in our examples assumed to
be in same folder as where program run
• Possible to specify path names
 Full path name
 Relative path name
• Be aware of differences of pathname
styles in different operating systems
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Methods of the Class File …
• Recall that a File object is a systemindependent abstraction of file's path
name
• Class File has methods to access
information about a path and the files in it
 Whether the file exists
 Whether it is specified as readable or not
 Etc.
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
… Methods of the Class File
• Figure 10.4 Some methods in class File
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Basic Binary-File I/O
•
•
•
•
Creating a Binary File
Writing Primitive Values to a Binary File
Writing Strings to a Binary File
The Class EOFException
• Programming Example:
Processing a File of Binary Data
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Creating a Binary File …
• Stream class ObjectOutputStream
allows files which can store
 Values of primitive types
 Strings
 Other objects
• View program which writes integers,
listing 10.4 class BinaryOutputDemo
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
… Creating a Binary File
Sample
screen
output
• Note the line to open the file
 Constructor for ObjectOutputStream
cannot take a String parameter
 Constructor for FileOutputSream can
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Writing Primitive Values to a Binary File…
• Method println not available
 Instead use writeInt method
 View in listing 10.4
• Binary file stores numbers in binary form
 A sequence of bytes
 One
immediately
after another
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
…Writing Primitive Values to a Binary File…
• Figure 10.5a Some methods in class
ObjectOutputStream
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
…Writing Primitive Values to a Binary File…
• Figure 10.5b Some methods in class
ObjectOutputStream
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
…Writing Primitive Values to a Binary File
• Figure 10.5c Some methods in class
ObjectOutputStream
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Writing Strings to a Binary File
• Use method writeUTF
• Example
outputStream.writeUTF("Hi Mom");
• UTF stands for Unicode Text Format
• Uses a varying number of bytes to store
different strings
 Depends on length of string
 Contrast to writeInt which uses same for each
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Reading from a Binary File…
• File must be opened as an
ObjectInputStream
• Read from binary file using methods which
correspond to write methods
 Integer written with writeInt will be read
with readInt
• Be careful to read same type as was
written
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
…Reading from a Binary File…
• Figure 10.6a Some methods of class
ObjectInputStream
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
…Reading from a Binary File…
• Figure 10.6b Some methods of class
ObjectInputStream
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
…Reading from a Binary File…
• Figure 10.6c Some methods of class
ObjectInputStream
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
…Reading from a Binary File…
• Figure 10.6d Some methods of class
ObjectInputStream
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
…Reading from a Binary File…
• Figure 10.6e Some methods of class
ObjectInputStream
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
…Reading from a Binary File
• View program to read, listing 10.5
class BinaryInputDemo
Sample
screen
output
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
The Class EOFException
…
• Many methods that read from a binary file
will throw an EOFException
 Can be used to test for end of file
 Thus can end a reading loop
• View example program, listing 10.6
class EOFExceptionDemo
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
… The Class EOFException
• Note the -1 formerly needed as a sentinel
value is now also read
Sample
screen
output
• Always a good idea to check for end of file
even if you have a sentinel value
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Programming Example
• Processing a file of binary data
 Asks user for 2 file names
 Reads numbers in input file
 Doubles them
 Writes them to output file
• View processing program, listing 10.7
class Doubler
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Binary-File I/O, Objects & Arrays
• Binary-File I/O with Objects of a Class
• Some Details of Serialization
• Array Objects in Binary Files
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Binary-File I/O with Class Objects…
• Consider the need to write/read objects
other than Strings
 Possible to write the individual instance
variable values
 Then reconstruct the object when file is read
• A better way is provided by Java
 Object serialization – represent an object as a
sequence of bytes to be written/read
 Possible for any class implementing
Serializable
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
…Binary-File I/O with Class Objects…
• Interface Serializable is an empty
interface
 No need to implement additional methods
 Tells Java to make the class serializable
(class objects convertible to sequence of
bytes)
• View sample class , listing 10.8
class Species
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
…Binary-File I/O with Class Objects…
• Once we have a class that is specified as
Serializable we can write objects to a
binary file
 Use method writeObject();
• Read objects with method readObject();
 Also required to use typecast of the object
• View sample program, listing 10.9
ClassObjectIODemo
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
…Binary-File I/O with Class Objects
Sample
screen
output
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Some Details of Serialization…
• Requirements for a class to be serializable
 Implements interface Serializable
 Any instance variables of a class type are
also objects of a serializable class
 Class's direct superclass (if any) is either
serializable or defines a default constructor
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
…Some Details of Serialization
• Effects of making a class serializable
 Affects how Java performs I/O with class
objects
 Java assigns a serial number to each object
of the class it writes to the
ObjectOutputStream
 If same object written to stream multiple
times, only the serial number written after first
time
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Array Objects in Binary Files…
• Since an array is an object, possible to
use writeObject with entire array
 Similarly use readObject to read entire
array
• View array I/O program, listing 10.10
class ArrayIODemo
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
…Array Objects in Binary Files
Sample
screen
output
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
Summary …
• Files with characters are text files
 Other files are binary files
• Programs can use PrintWriter and
Scanner for I/O
• Always check for end of file
• File name can be literal string or variable
of type String
• Class File gives additional capabilities
to deal with file names
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved
… Summary
• Use ObjectOutputStream and
ObjectInputStream classes enable
writing to, reading from binary files
• Use writeObject to write class
objects to binary file
• Use readObject with type cast to read
objects from binary file
• Classes for binary I/O must be serializable
JAVA: An Introduction to Problem Solving & Programming, 5th Ed. By Walter Savitch and Frank Carrano.
ISBN 0136130887 © 2008 Pearson Education, Inc., Upper Saddle River, NJ. All Rights Reserved